Alunite specimen, Brigham Young University geology collection. — Photo: Andrew Silver · Public domain
Alunite
Also known as: Alum stone.
Alunite is a hydroxylated aluminium potassium sulfate mineral, formula KAl3(SO4)2(OH)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum. First called aluminilite by J.C. Delamétherie in 1797, this name was contracted by François Beudant three decades later to alunite. Alunite crystals morphologically are rhombohedra with interfacial angles of 90° 50', causing them to resemble cubes. Crystal symmetry is trigonal. Minute glistening crystals have also been found loose in cavities in altered rhyolite.
Intro adapted from Wikipedia (CC BY-SA).
Alunite is an IMA-recognized mineral species in the sulfate class, with the chemical formula KAl₃(SO₄)₂(OH)₆.
It crystallizes in the hexagonal crystal system (space group R-3m). It is a member of the Alunite-Alunite structural group.
Key properties: Mohs hardness 3.5–4; specific gravity 2.6–2.9; typically white or gray or yellow or pink or red; White streak; Vitreous, somewhat pearly on {0001} luster; refractive index 1.582; optically uniaxial.
IMA symbol Alu, IMA status: approved, named in 1565.
Formed between 15 ° C and 400 ° C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification.
It is found associated with Kaolinite, halloysite, diaspore, pyrite, gypsum, quartz.
A contraction of the earlier name aluminilite.
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